\documentclass{article}
\usepackage{booktabs}
\usepackage{tabularx}
\usepackage[margin=1in]{geometry}
\begin{document}

\begin{table}[tbp] \centering
\newcolumntype{R}{>{\raggedleft\arraybackslash}X}
\newcolumntype{L}{>{\raggedright\arraybackslash}X}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\begin{tabularx}{\linewidth}{@{}lCCCC@{}}

\toprule
{out\_var}&{e5\_all}&{diesel\_all}&{e5\_20to100}&{diesel\_20to100} \tabularnewline
\midrule \addlinespace[\belowrulesep]
Coefficient&-.024&-.014&-.025&-.014 \tabularnewline
Standard error&.0004&.0004&.0009&.0013 \tabularnewline
95\% CI LB&-.025&-.015&-.027&-.016 \tabularnewline
95\% CI UB&-.023&-.013&-.023&-.011 \tabularnewline
Observations&647500&768550&50400&56500 \tabularnewline
\bottomrule 

\end{tabularx}
\end{table}
\end{document}
